This invention relates to aromatic polymers. In particular, this invention relates to aromatic isoimide polymers.
Considerable research effort has been directed toward the synthesis of rigid rod polymers for their unique ordering properties that provide extremely high modulus/high strength films and fibers. One class of materials of particular interest is the aromatic heterocyclic bisbenzazole polymers. These polymers exhibit excellent thermal and thermooxidative stabilities. Another class of materials having comparable high temperature properties is the aromatic polyimides. The polyimides are attractive, not only for their high temperature properties, but also because of the low cost of the diamine and dianhydride monomers used in their synthesis.
An aromatic polymide with the desired para-ordered geometry and be prepared from pyromellitic dianhydride (PMDA) and p-phenylene-diamine. High molecular weight polyamic acid has been prepared in dimethylacetamide (DMAC) using these monomers; however, thermal or chemical cyclodehydration leads to an insoluble, infusible material. Fabrication of this material is normally carried out via the DMAC-soluble polyamic acid which produces two units of water per repeat unit during high temperature thermal cyclodehydration to the imide structure. The water produced by this process limits the utility of this material, particularly in the fabrication of thick components.
It is known that certain polyisoimides can be used to form the corresponding polyimides by thermal curing. Such polyisomides may be prepared by reacting a carboxylic acid dianhydride with a tetravalent aromatic diamine to produce a polyamic acid, and treating the resulting polyamic acid with a dehydrating agent to produce the corresponding polyisoimide. The polyisoimide to polyimide route is attractive from the standpoint that in the course of thermal curing no water vapor is released which could cause voids or defects in thick components. In general, the soluble polyisoimides prepared from aromatic diamines disclosed in the prior art are not linear, i.e., that portion of the polymer backbone contributed by the diamine is either not para-oriented with respect to the amino groups, or contains a non-linear constituent.
As mentioned previously, the aromatic polyimide prepared from pyromellitic dianhydride and p-phenylene diamine has the desired para-ordered geometry. This polymer has attractive high temperature properties and low cost. Unfortunately, when prepared via the soluble polymic acid route, the utility of the polymer is limited because of the water produced in the thermal cyclodehydration step.
We attempted to prepare a polyimide from pyromellitic dianhydride and p-phenylene diamine via the polyisoimide to polyimide route. We found that the polyisoimide prepared from these monomers was insoluble in all the solvents tested.
Accordingly, it is an object of the present invention to provide soluble polyisoimides which can be thermally rearranged to para-ordered polyimides.
It is another object of the present invention to provide a method for preparing soluble polyisoimides.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art.